Paper accepted for publication in J. National Agriculture
Society of Sri Lanka

Research Report MAHA 1993/94, RARC, Aralaganwila

1.
Name of Researcher(s)K.H.S Peiris (RO),

A.S.P Weerasinghe (RO)
W.M.U.B. Wickrama (RA)

2.
Title of Experiment Mango Flowering
studies with Paclobutrazol

3.
Objective(s)

To assess the efficacy of Paclobutrazol for flowering
enhancement of Mango

4.
Crop Mango

5.
Location RARC, Aralaganwila

ARC, Girandurukotte and
HRF, Ulpothagama

6.
Season MAHA 1993/94

7.
Methodology

A number of trials were conducted
at RARDC, Aralaganwila, ARC, Girandurukotte and HRF, Ulpothagama to assess
the feasibility of using Paclobutrazol to improve flowering and productivity
of several mango varieties. The observations and results are presented
separately for each trial.

The product used in this
experiment was Cultar, a commercial formulation marketed by ICI, London, having 25%
v/v of Paclobutrazol.

8.
Results

Trial
# 1. RARDC, Aralaganwila.

Two trees each from
varieties Willard and Malwane were treated with Paclobutrazol in June 1993.
Six milliliters of Paclobutrazol was applied in 1.0 L of water as a collar
drench. Thus each tree received 24 ml of Cultar (25 % v/v Paclobutrazol).
Plants were observed closely for flowering, flowering intensity, flowering
periods, flower morphology, fruit yield and characteristics of subsequent
vegetative flushes.

The yield data for
experimental trees are given in Table 1. Paclobutrazol treated Willard trees
had high intensity of flowering while untreated tree had extensive vegetative
flushing during fruit development. The highest fruit yield given by untreated
tree is due to its large size. Tree number 3 produced few fruits as its
flowering was delayed until September. This tree flowers latter in every year Paclobutrazol
did not advanced its flowering.

Few days after Paclobutrazol
application to Willard trees, leaves of treated trees had
turned to dark green in color compared to the control tree. During flowering Paclobutrazol
treated trees produced shorter panicles compared with panicles
in untreated trees. The vegetative flush in following year showed more
compact appearance than that of flushes in control trees. The dwarfing nature
of flushes and leaf wrinkling was more pronounced in smaller tree applied
with 6 ml of Paclobutrazol.

These differences in leaf color, panicle size and subsequent
flushes are the evidence that Paclobutrazol has been taken up by the tree and
moved to apical meristems where it exert its physiological effects. However,
this kind of morphological changes were not observed in Malwane trees. Though
Malwane trees did not flower in July or September, 1993 as observed in
variety Willard, only one tree flowered in early April, 1994 and again in mid
June, 1994 for the second time. However, though this tree fully flowered all
developing fruits dropped prematurely. Apart from these two treated trees
there were 25 more untreated Malwane trees that did not flower at all.
Therefore, this observation suggest that even if Paclobutrazol induce
flowering in unproductive varieties, trees can not support the growth of
fruits possibly may be due to low levels of carbohydrate reserves in storage
to support fruit development.

What is interesting
here to note is that in variety Willard number of fruits as well as fruit
number per unit canopy volume is higher in tree treated with Paclobutrazol that flowered in July. Canopy volume was calculated by using canopy width,
breadth and height measurements assuming a conical shape of all canopies. The
lower yields given by the tree flowering in September is due to bad weather conditions
prevailed during flowering time.

Trial # 2: ARC, Girandurukotte

Two trees each from
six exotic mango varieties were treated with 6 ml of Paclobutrazol in June
1993. The varieties used for this experiment were Joe Welch, Sensation,
Kensington, Pope summer, Tommy Atkins and Haden. When the treatments were
applied, the trees wereapproximately
3.5 years old and two varieties, Joe Welch and Sensation had already started
to bear few fruits. Observations indicated no signs of Paclobutrazol activity
in trees of any variety tested as displayed by variety Willard in response to Paclobutrazol
application at Aralaganwila. However, varieties Sensation and Joe Welch produced few fruits, both in treated as well as untreated trees
showing that the bearing is not as a result of Paclobutrazol application.

Trial
# 3: HRF, Ulpothagama.

Three rates of Paclobutrazol (0,
4 and 6 ml/tree) were tested for three mango varieties, Willard, Karuthakolomban
and Velleikolomban. This experiment was conducted in a
Randomized Complete Block Design having four replicates with one tree per
replicate. Plants were blocked according to tree size. Treatments were
applied in June, 1993. Table 2. list the tree number for each variety and
replicate treated with different Cultar rates

Flowering intensity and fruit
yield records were taken to evaluate the effects of Paclobutrazol on
flowering and fruit yield of trees. Visual counts of flowering intensity, (
approximate % of shoots flowered on the canopy) were recorded on September
22, 1993, when peak flowering was observed in mangoes at Ulpothagama. Fruits
were harvested from late December through mid January.

Table
2. Tree numbers for each replicate and Cultar treatments for different
varieties.

------------------------------------------------------------

Treatment

WillardVelleikolombanKaruthakolomban

------------------------------------------------------------

Control

R1 -
405R1 - 188R1 - 264

R2 -
406R2 - 197R2 - 265

R3 -
390R3 - 199R3 - 262

R4 -
392R4 - 200R4 - 299

4
ml/tree

R1 -
397R1 - 216R1 - 254

R2 -
398R2 - 208R2 - 259

R3 -
399R3 - 209R3 - 263

R4 -
407R4 - 218R4 - 260

6
ml/tree

R1 -
394R1 - 248R1 - 250

R2 -
395R2 - 249R2 - 246

R3 -
396R3 - 230R3 - 247

R4 -
391R4 - 228R4 - 251

------------------------------------------------------------

Table
3. Flowering intensity (% shoots flowering)

------------------------------------------------------------

TreatmentVariety

(ml,PBZ/Tree)WLDKCVC

------------------------------------------------------------

012.58.740.0

472.525.090.0

687.548.796.7

------------------------------------------------------------

Paclobutrazol had a clear effect
on enhancement of flowering in all three varieties (Table 3). Results
demonstrated that at the rates tested in this experiment, Karuthakolomban trees did not attain full flowering like in varieties Willard and
Velleikolomban. Application rates more than 6 ml per tree must be further
tested for Karuthakolomban.

Fruit yields of varieties Willard
and Karuthakolomban are presented in Tables 4 and 5. Even though four trees
were used for each treatment some trees had to be removed from each treatment
as a result of some mistakes made in recording yield data. Velleikolomban yields are not presented as fruit yield data records were not taken properly
in this season for that variety. Number of trees used for calculating mean
values are given against each treatment.

Table
4. Fruit yield of variety Willard.

---------------------------------------------------------------------

TreatmentNo of FruitsFruit WeightFruit

(ml,PBZ/Tree)Treesper
treeper tree (kg)Size (g)

---------------------------------------------------------------------

04296.1211

43327.5232

6216432.5198

---------------------------------------------------------------------

Table
5.Fruit yield of variety Karuthakolomban.

---------------------------------------------------------------------

TreatmentNo of FruitsFruit WeightFruit

(ml,PBZ/Tree)Treesper
treeper tree (kg)Size (g)

---------------------------------------------------------------------

03176.7394

43259.0360

625320.3383

---------------------------------------------------------------------

Results on yield figures for both
varieties unquestionably indicate that Paclobutrazol directly improve the
fruit yields. However, with increasing fruit number average fruit weight has
been slightly diminished.

9. Remarks

The experimental data and
observations revealed that Paclobutrazol is a very valuable growth regulator
to be used in mango production. Apart from mango, it has many uses in other
fruit species such as Citrus, avocado etc. and vegetables as well as in ornamental
crops.

In relation to the effects of Paclobutrazol
on mango, it seems that Paclobutrazol influences depend on
mango variety. Variety Willard seems to be very sensitive to Paclobutrazol as
shown by its immediate uptake and its effects on enhancing flowering and
fruit yield. Varieties Karuthakolomban and Velleikolomban also showed good
response to Paclobutrazol application. Further information is required to
assess the exact quantities to be applied because it seems that the optimum
responses are given under a narrow concentration range. Experimental
applications are necessary to get a clear idea, as quantities to be applied
are determined by variety, tree size (rather than tree age) and soil type.

10.Summary

A
number of experiments conducted to find out the efficacy of Cultar
(Paclobutrazol) on mango flowering indicated that Cultar enhance the
flowering of mango varieties Willard, Velleikolomban and Karuthakolomban.
However, this growth regulator did not influence flowering of some exotic
varieties. Though Cultar promoted flowering of variety Malwane, fruit set was
very poor. Among the responsive varieties, Willard showed a great sensitivity
to this growth regulator.

Mango yields in a particular
fruiting season to a greater extent depend on flowering intensity of trees.
Best yields are obtained when most of the shoots flower without any
development of growth flushes from flowering to maturity, when trees of an
appropriate variety are well managed during the period prior to flower
initiation. However, even with the employment of a sound tree management
package, it is a common phenomena that under most circumstances vegetative flushes
occur during the period from flowering to fruit maturity. Vegetative flushes
at this time affect fruit set and fruit size because growing vegetative
flushes have the highest competitive demand for tree reserves, and as a
result more fruits drop prematurely and remaining fruits become smaller.
Therefore, methods to control vegetative flushes at flowering may support
increasing yields.

Paclobutrazol
(Cultar) or P333 is a plant growth regulator showing much promise in
controlling vegetative growth in fruit trees and thereby enhancing flowering,
fruit set and fruit yields. This chemical belonging to the category of growth
retardants. Paclobutrazol has been tested on mangoes in other countries and
results are very encouraging. Results indicate that while enhancing flowering
and fruit yield, it also keeps the trees dwarf. This is a good characteristic
for fruit trees as it facilitates better orchard management, with a higher
plant density for improving orchard productivity.

Trials have been
conducted since June 1993 to study the effect of Paclobutrazol on mango
productivity. Preliminary results of these trials are presented in this
report.

Methodology

A number of trials
were conducted at RARDC, Aralaganwila, ARC, Girandurukotte and HRF,
Ulpothagama to assess the feasibility of using Paclobutrazol to improve
flowering and productivity of several mango varieties. The observations and
results are presented separately for each trial.

Trials conducted in
Australia have shown that best results are obtained when 4 - 6 ml of Paclobutrazol
is applied to 5-7 year old trees. In trials testing different
methods of application, results have shown that application as a collar
drench around the base of the tree is far superior to all other application
methods such as foliar spray, trunk injection, soil application to root zone
as band applications or complete spreading around the root zone etc.(
Winston, 1992). Therefore, in all trials going to be presented here
experimental dose of Paclobutrazol was applied as a collar drench in one
liter of water. The product used in this experiment was Cultar, a commercial
formulation marketed by ICI, London, having 25% v/v of Paclobutrazol.

Trial
# 1. RARDC, Aralaganwila

Two trees each from
varieties Willard and Malwane were treated with Paclobutrazol in June 1993.
Six ml of Paclobutrazol was applied in 1.0 L of water as a collar drench.
Thus each tree received 24 ml of Cultar (25 % v/v Paclobutrazol). Plants were
observed closely for flowering, flowering intensity, flowering periods,
flower morphology, fruit yield and characteristics of subsequent vegetative
flushes.

The yield data for
experimental trees are given in Table 1. Paclobutrazol treated Willard trees
had high intensity of flowering while untreated tree had extensive vegetative
flushing during fruit development. The highest fruit yield given by untreated
tree is due to its large size. Tree number 3 produced few fruits as its
flowering was delayed until September. This tree flowers latter in every year Paclobutrazol
did not advanced its flowering.

Few days after Paclobutrazol
application to Willard trees, leaves of treated trees had
turned to dark green in color compared to the control tree. During flowering Paclobutrazol
treated trees produced shorter panicles compared with panicles
in untreated trees. The vegetative flush in following year showed more
compact appearance than that of flushes in control trees. The dwarfing nature
of flushes and leaf wrinkling was more pronounced in smaller tree applied
with 6 ml of Paclobutrazol.

These differences in
leaf color, panicle size and subsequent flushes are the evidence that Paclobutrazol
has been taken up by the tree and moved to apical meristems
where it exert its physiological effects. However, this kind of morphological
changes were not observed in Malwane trees. Though Malwane trees did not
flower in July or September, 1993 as observed in variety Willard, only one
tree flowered in early April, 1994 and again in mid June, 1994 for the second
time. However, though this tree fully flowered all developing fruits dropped
prematurely. Apart from these two treated trees there were 25 more untreated
Malwane trees that did not flower at all. Therefore, this observation suggest
that even if Paclobutrazol induce flowering in unproductive varieties, trees
can not support the growth of fruits possibly may be due to low levels of
carbohydrate reserves in storage to support fruit development.

What is interesting
here to note is that in variety Willard number of fruits as well as fruit
number per unit canopy volume is higher in tree treated with Paclobutrazol that flowered in July. Canopy volume was calculated by using canopy width,
breadth and height measurements assuming a conical shape of all canopies. The
lower yields given by the tree flowering in September is due to bad weather
conditions prevailed during flowering time.

Trial
# 2: ARC, Girandurukotte

Two trees each from six
exotic mango varieties were treated with 6 ml of Paclobutrazol in June 1993.
The varieties used for this experiment were Joe Welch, Sensation, Kensington,
Pope summer, Tommy Atkins and Haden. When the treatments were applied, the
trees wereapproximately 3.5 years
old and two varieties, Joe Welch and Sensation had already started to bear few
fruits. Observations indicated no signs of Paclobutrazol activity in trees of
any variety tested as displayed by variety Willard in response to Paclobutrazol
application at Aralaganwila. However, varieties Sensation and Joe Welch produced few fruits, both in treated as well as untreated trees
showing that the bearing is not as a result of Paclobutrazol application.

Trial
# 3: HRF, Ulpothagama

Three rates of Paclobutrazol
(0, 4 and 6 ml/tree) were tested for three mango varieties,
Willard, Karuthakolomban and Velleikolomban. This experiment was conducted in
a Randomized Complete Block Design having four replicates with one tree per
replicate. Plants were blocked according to tree size. Treatments were
applied in June, 1993.

Flowering intensity
and fruit yield records were taken to evaluate the effects of Paclobutrazol on flowering and fruit yield of trees. Visual counts of flowering intensity,(
approximate % of shoots flowered on the canopy) were recorded on September
22, 1993, when peak flowering was observed in mangoes at Ulpothagama. Fruits
were harvested from late December through mid January.

Table
2. Flowering intensity (% shoots flowering)

Treatment

(ml,PBZ/Tree)

Variety

WLD

KC

VC

0

12.5

8.7

40.0

4

72.5

25.0

90.0

6

87.5

48.7

96.7

Paclobutrazol had a
clear effect on enhancement of flowering in all three varieties (Table 2).
Results demonstrated that at the rates tested in this experiment, Karuthakolomban
trees did not attain full flowering like in varieties Willard
and Velleikolomban. Application rates more than 6 ml per tree must be further
tested for Karuthakolomban.

Fruit yields of
varieties Willard and Karuthakolomban are presented in Tables 3 and 4. Even
though four trees were used for each treatment some trees had to be removed
from each treatment as a result of some mistakes made in recording yield
data. Velleikolomban yields are not presented as fruit yield data records
were not taken properly in this season for that variety. Number of trees used
for calculating mean values are given against each treatment.

Table
3. Fruit yield of variety Willard

Treatment

(ml,PBZ/Tree)

No of

Trees

No of Fruits

per tree

Fruit Weight

per tree (kg)

Fruit

Size (g)

0

4

29

6.1

211

4

3

32

7.5

232

6

2

164

32.5

198

Table
4. Fruit yield of variety Karuthakolomban

Treatment

(ml,PBZ/Tree)

No of

Trees

No of Fruits

per tree

Fruit Weight

per tree (kg)

Fruit

Size (g)

0

3

17

6.7

394

4

3

25

9.0

360

6

2

53

20.3

383

Results on yield
figures for both varieties unquestionably indicate that Paclobutrazol directly improve the fruit yields. However, with increasing fruit number
average fruit weight has been slightly diminished.

Discussion

The experimental data
and observations revealed that Paclobutrazol is a very valuable growth
regulator to be used in mango production. Apart from mango, it has many uses
in other fruit species such as Citrus, avocado etc. and vegetables as well as
in ornamental crops.

In relation to the
effects of Paclobutrazol on mango, it seems that Paclobutrazol influences
depend on mango variety. Variety Willard seems to be very sensitive to Paclobutrazol
as shown by its immediate uptake and its effects on enhancing
flowering and fruit yield. Varieties Karuthakolomban and Velleikolomban also
showed good response to Paclobutrazol application. Further information are
required to assess the exact quantities to be applied because it seems that
the optimum responses are given under a narrow concentration range.
Experimental applications are necessary to get a clear idea, as quantities to
be applied are determined by variety, tree size (rather than tree age) and
soil type.

Observations also
indicate that Paclobutrazol can do very little to increase fruit yields of
un-adaptable varieties of mango. Though, in some instances Paclobutrazol induces the trees to flower, poor fruit set and severe premature fruit drop
limit the fruit yields of such varieties. This may be as a result of
insufficient tree reserves to support growth and development of fruits under
such conditions.

In previous trials
conducted by Dissanayake (1992) off-season flowering of mango variety Karuthakolomban
has been successful in wet zone yielding 11.8 - 23.5 kg of
fruits per tree in November at Walpita. However, our observations at
Aralaganwila and Ulpothagama indicate that effects are more pronounced in
enhancement of flowering and fruit set rather than on off-season flower
induction to get fruits at any time of the year. Observations have confirmed
that during the normal season Paclobutrazol treated trees flower profusely
and produce higher yields compared to control trees. Little off-season
flowering was also observed, but fruit set was very poor as most of the
flowers dried out as a result of inclement weather conditions during the time
of flowering. This is however good to happen in such a way as it will not
affect over exploitation of trees which is an inherent problem with flower
induction technology as observed in the Philippines now. Philippines mango
growers has now felt this problem and expressed their concerns about the
declining tree productivity with the use of potassium nitrate technology for
off-season flowering.

Another concern in
using new agricultural chemicals is issues in relation to their toxicology,
environmental and residual effects. Paclobutrazol is of low acute toxicity,
the oral LD50 to rat being 1300 - 2000 mg/kg. This chemical is rapidly
excreted by mammals and does not accumulate in the tissues. It is not
mutagenic. Paclobutrazol is also of low toxicity to fish, birds, bees,
earthworms and soil microorganisms.

Residues have not
been detected in fruits after soil applications, even where large reductions
of vegetative growth have been observed. When foliar applications were made 2
weeks before harvesting on apples at 1 kg/ha, residues at harvest have been
found to be below 0.5 ppm. The half-life of Paclobutrazol varies with soil
type and climatic conditions, but is generally between 6 and 12 months. Paclobutrazol
has a low mobility in soils (Helling Class 2) so residues should not
constitute a leaching risk.

In the final analysis, it is
evident that Paclobutrazol can be integrated into mango tree management
systems provided that other cultural practices are well adopted to sustain
the yield increases with out affecting the long term tree productivity.
Introduction of Paclobutrazol to growers requires good extension effort and
technical support to successfully transfer this useful technology (Voon et al,
1992). If this technology is properly employed, orchard productivity and
profitability could be significantly improved.

Mr.
Bruce Spake: How safe is mango treated with Paclobutrazol for human
consumption?

A.

I have not come across any reports indicating that Paclobutrazol is harmful to humans. Paclobutrazol is being used commercially in other
countries as well on a variety of fruit crops including apple, pear and
avocado etc., in addition to mango. Uses are many more in ornamental
horticulture.

Paclobutrazol is a
plant growth regulator affecting the bio-synthesis of Gibberellins, a class
of natural hormones synthesized by plants to promote vegetative growth.
Paclobutrazol molecules irreversibly and very competitively couples with two
enzymes involved in Gibberellins synthesis. This reaction is very specific
only to those two enzymes found in plant systems. Being a plant growth
regulator, Paclobutrazol is active only in plant systems, but not in animal
systems like in the case if insecticides. We must not be too alarmed toward
the use of agricultural chemicals as far as those are judiciously used to
harness their potential applications. When Paclobutrazol is commercially used
it shows us that it has passed all rigorous FDA tests in USA in order to
utilize it for agricultural purposes.

[ Paper
presented at Mango Symposium held in July 3-5 at Hotel Seruwa, Polonnaruwa.

Research Report 2000, FCRDI, Maha
Illuppallama

Flower induction studies for
off-season fruit production in mango

K.H.S. Peiris, RO,
and A.G. Premachandra, RSA.

Introduction

Mangoes in Sri Lanka flower twice a year. Generally,
majority of the trees in Wet and Intermediate Zones flower in February - March
for the Yala crop harvested in June- July which is considered as the main
crop for the above mentioned agro-climatic zones. Trees in the Dry Zone
flower July through September for the main Maha crop harvested in
November-January. Since mangoes are mostly cultivated as a rain fed crop,
timing of the seasons varies however, depending on rainfall distribution
patterns.

Production drops in between the two seasons. Prices of
fruit go up. Also production gluts during the main season cause heavy losses
and price of fruit fall. Many small scale growers can not sell their produce
during the peak season. To overcome these marketing problems, a package of
technology to produce fruit in the off-season is a must.

A package of technologyfor off season mango production needs 1. A technique to get the
leaf flush that emerge after harvesting mature without additional flushes and
2. a technique to induct flowering in mature shoots. Apart from the above
growers must have irrigation facilities to facilitate growth and development
of fruit after flowering. A good pest and disease management program is
another important requirement to get a good crop because pest and disease
pressure is heavy in off-season.

Experiments were conducted to identify mango varieties
that responds to the application of Thiourea, To determine the effect of
Paclobutrazol on mango flowering with the objectives of identifying varieties
and suitable rates of application.

Materials and methods

An observation study was conducted at FCRDI, Maha Illuppallama,
with 14 mango varieties to ascertain the efficacy of Thiourea as a flower
inducer. A solution of 0.75 % Thiourea was sprayed to half of the tree canopy
in July 1999. All selected trees (one tree per variety) had mature shoots
suitable for flower induction. Flowering was observed three weeks after
application of Thiourea.

Another experiment was conducted at the nursery of the
Government Seed Farm, Maha Illuppallama to study the effect of Paclobutrazol
and Thiourea on flowering of Mango cv. Willard. Three rates of Paclobutrazol
( 0.00, 0.75 and 1.00 g/m of canopy diameter) and three rates of Thiourea
(0.00, 0.50 and 1.00%) were tested in a two factor factorial in RCBD with
four replicates. Trees were blocked according to tree size as measured by stem
girth at 30 cm above the ground. Trees in the experimental block had a major
flush in October 1999. Trees selected for this experiment had more than 80%
of the shoots as new flush. After the leaf flush fully expanded,
Paclobutrazol was applied as a collar drench around the base of the tree, in
December 08, 1999. It was planned to apply Thiourea to induce flowering 3
months after Paclobutrazol application. However, trees started to flower
naturally in third week of January, 2000. Almost all flowers were destroyed
due to severe anthracnose infestation. Then trees were sprayed with Thiourea
solution in March 22, 2000. Flowering was observed again in April, 2000.
Flowering intensity was visually assessed as % of the flowering shoots in
canopy.

Results and Discussion

Observation study conducted to
ascertain the efficacy of Thiourea as a mango flower inducer showed that out
of 14 varieties, only four varieties, Carabao, Kohu amba, Nam Doc Mai and
Willard responded positively (Table 1). Kohu amba tree used in this
experiment was a seedling tree and a second application of Thiourea to the
responsive varieties in the following year further confirmed that Thiourea
can be effectively used to induce flowering of mature shoots of these
varieties.

Trees used in the replicated
experiment flowered naturally in 3rd week of January, 2000, nearly 6 weeks
after Cultar application. Trees treated with Cultar showed a high flowering
intensity as measured by % shoots flowered on canopy. Due to the prevailing
rainy weather at that time, almost all flowers were dropped due to severe anthracnose
infection. These trees were treated with Thiourea in March 22,
2000. Flowering was again observed in second week of April. Again, trees
treated with Cultar had higher flowering intensity. Due to the prevailing
humid rainy weather conditions, most of the flowers were dropped due to anthracnose
resulting a very low fruit set.

It is apparent that application
of Cultar at the end of leaf expansion improve flowering of mango cv Willard
(Table 2). Effect of Thiourea was not discernible due to complicated early
flowering observed in this season. It was clearly noted, however, that length
of panicles become very short in trees treated with 1.00g/m of Cultar. There
was no significant difference in flowering intensity between the two rates
tested indicating that 0.75 g/m of Cultar is sufficient for mango cv.
Willard.

It is a common phenomenon that
with flowering some of the shoots of mango canopy flush. It was noted that
trees treated with Cultar have shown a lower flushing intensity (Table 3.).

Flower induction studies for off-season fruit production in mango

K.H.S. Peiris, RO,
and S. A. Senevirathna, RA

Introduction

Mango production in Sri Lanka
is seasonal. Majority of trees in the Wet and Intermediate Zones flowers in
January through March for the Yala crop harvested in May - July that is
considered as the main crop for above-mentioned agro-climatic zones. Trees in
the Dry Zone flower July through September for the main Maha crop harvested
in November-January. Mangoes are mainly cultivated as a rain-fed crop, thus
the time of flowering and harvesting may shift considerably depending on
rainfall distribution patterns. Abundant production of in-season mangos
within short period of time leads to low prices. Production drops in between
the two seasons resulting in higher prices. Therefore, an off-season mango
production technique is very important for production management of mango.

Reliable flowering is necessary
to obtain consistent production of mango under tropical climatic conditions.
Flowering from one season to the other is unreliable because of the
environmental signals for flower initiation is often inconsistent, subtle or
poorly defined. Understanding of the factors controlling flower bud initiation
in mango trees has been very limited. An alternative to dependence upon
environmental signals for flower initiation is the development of management
strategies that can substitute these signals.

Though growth regulators have
been tested for promoting/inhibiting flower production in mango in different
countries, their effects have been limited to certain cultivars and
geographical locations. Therefore, growth regulator chemicals and application
rates that can promote or induce flowering must be tested under local
conditions for the locally available mango varieties. The objectives of these
studies were to study the influence of application rate of Paclobutrazol on
enhancement of flowering and fruit production in mango.

Materials and Methods

A field experiment was
conducted at Government Seed Production Farm, Maha Illuppallama, to study the
effect of rate of application of Paclobutrazol on flowering of mango
cultivars Velleikolomban and Willard. Three rates of Paclobutrazol (0.00,
0.50 and 0.75 g a.i. /m of canopy diameter for cv Willard and 0.00, 0.75 and
1.25 g a.i. /m for cv Velleikolomban ) were tested. The formulation used was
a wettable powder with 10% Paclobutrazol. The amount to be applied to each
tree was dissolved in 1.5 l of water and applied as a collar drench around
the base of tree at about one month after leaf bud break when the leaf flush
was fully expanded.

Experiment was conducted in a
RCBD with six single-tree replicates. Trees were 8 years old. Experiment was
blocked according to tree size as measured by stem girth at 30 cm above the
ground. Mango trees selected for this experiment had more than 80% of the
shoots in the canopy as new flush.

Flowering was assessed as the %
of the shoots flowering using randomly selected 20 tagged shoots. Panicle
length was measured after full bloom using three panicles taken at random.
Data collected were analyzed by analysis of variance in RCBD. For treatment
mean comparison LSD values were calculated (P=0.05).

An observation study was conducted
in farmers’ fields at Upuldeniya, Anuradhapura to assess the effect of
Paclobutrazol on mango productivity. Paclobutrazol was applied as a soil
drench when the major leaf flush fully expanded at the rate of 1.00 g a.i/m
for mango cultivars Karuthakolomban and Velleikolomban and 0.75 g a.i/m for
cv Willard. Five trees of Karuthakolomban and three trees each of Velleikolomban
and Willard were treated with Paclobutrazol. An equal number
of trees were used as controls. Trees used for this experiment was 12 years
old. After flowering, Imidacloprid (10 ml/10 l water) was sprayed to flower
buds before full bloom to control Mango Hopper damage to flowers. Total
number of fruits harvested in the following season was recorded and mean
number of fruits per tree and standard deviation calculated for each
treatment in each variety.

Results and Discussion

Mango trees treated with
Paclobutrazol showed early bud break about two- weeks before the control
trees. Flowering intensity in terms of the percentage of shoots flowered and
lengths of flower panicles are shown in Table 1. Paclobutrazol enhanced the
flowering intensity of both mango cultivars. There was no difference in the
level of flowering between the two application rates used for both varieties.
It is a common phenomenon that with flowering, some of the shoots of mango
canopy produce leaf flush. It was noted that trees treated with Paclobutrazol
had only a few shoots with new flush.

Paclobutrazol shortened panicle
length significantly (Table 1) and the compacted flowers were visibly noted
in treated trees. However, the rates used did not compact the flowers too
much. Shorter flower panicles give an indication of the fact that
Paclobutrazol has been taken up by the trees.

Number of fruits produced in
mango cultivars Karuthakolomban, Velleikolomban and Willard were higher when
trees were treated with Paclobutrazol (Table 2). Variation in number of
fruits produced per tree was also lower with Paclobutrazol application. In
all three varieties, flower bud break commenced one to two weeks before the
flower bud break in control trees that were in close proximity to treated
trees. This shows that Paclobutrazol promote early flowering as well.

Conclusions

Paclobutrazol applied as
a color drench to mango cultivars Karuthakolomban, Velleikolomban and Willard
after the major leaf is flush fully expanded increased flowering intensity
and number of fruits per tree.

ABSTRACT

Experiments were conducted to evaluate Paclobutrazol
{(+/-)-(R*,R*)-B-[(4-chlorophenyl)methyl]-alpha-(1,1-dimethylethyl)-1H-1,2,4-triazole-1-ethanol} andThiourea for off-season flowering in
mango. Paclobutrazol applied at 0.50 - 1.25 g a.i./m of canopy diameter as a
soil drench promoted early and uniform flowering in mango cultivars Willard
and Velleikolomban. Application of Paclobutrazol also reduced the leaf flush
emerged at flowering. On-farm observation trials with mango cultivars Karuthakolomban, Willard and
Velleikolomban showed that Paclobutrazol
increased fruit yield while yield variation among trees were lower in the
treated trees than in the control trees. Thiourea applied as a foliar spray
of 0.50 % aqueous solution to mature shoots induced flowering in mango
varieties Carabao, Kohu-amba, Nam Doc Mai and Willard 11-21 days after
Thiourea application. These observations suggest that Paclobutrazol and Thiourea
can be successfully used to develop an off-season fruit production
technology for some mango varieties.

Keywords: Mango flowering; Off-season flowering; Growth
regulators

INTRODUCTION

Mango production in Sri Lanka is seasonal. Majority of
trees in the Wet and Intermediate Zones flowers in January through March for
the Yala crop harvested in May - July that is considered as the main crop for
above-mentioned agro-climatic zones. Trees in the Dry Zone flower July
through September for the main Maha crop harvested in November-January.
Mangoes are mainly cultivated as a rain-fed crop, thus the time of flowering
and harvesting may shift considerably depending on rainfall distribution
patterns. Abundant production of in-season mangos within short period of time
leads to low prices. Production drops in between the two seasons resulting in
higher prices. Therefore, an off-season mango production technique is very important
for production management of mango.

Reliable flowering is necessary to obtain consistent
production of mango under tropical climatic conditions. Flowering from one
season to the other is unreliable because of the environmental signals for
flower initiation is often inconsistent, subtle or poorly defined.
Understanding of the factors controlling flower bud initiation in mango trees
has been very limited. An alternative to dependence upon environmental
signals for flower initiation is the development of management strategies
that can substitute these signals.

A number of
diverse chemicals that have growth regulating properties in plants have been
tested for promoting/inhibiting flower production in mango in different
countries (Chacko, 1991). Paclobutrazol, a strong gibberellin biosynthesis
inhibitor, has been reported to be effective in promotion of flowering in
many fruit crop species including mango (Burondkar and Gunjate, 1991;
Charnvichitet al., 1991; Dissanayake,
1989; Hiller and Rudge, 1991; Jose and Roboucas, 2000; Kulkarni, 1988;
Peiris, 2001; Tongumpai et al., 1989; Tongumpai et al., 1991; Tongumpai et
al., 1997; Winston, 1992). Paclobutrazol as soil drench and foliar spray were
effective for promotion of flowering in mango, however, soil drench was more
significant, convenient and cost effective (Burondkar and Gunjate, 1991;
Winston, 1992). Application of 0.75 and 1.00 g a.i./m of canopy diameter of
Paclobutrazol to mango cv Willard increased flowering intensity
significantly. However, flowering intensity was not different between the two
concentrations tested showing that even lower rates of application would be
able to promote profuse flowering in cv Willard (Peiris, 2000).

Though growth regulators have been tested for
promoting/inhibiting flower production in mango in different countries, their
effects have been limited to certain cultivars and geographical locations
(Chacko, 1991). Therefore, growth regulator chemicals and application rates
that can promote or induce flowering must be tested under local conditions
for the locally available mango varieties. The objectives of these studies
were to study the influence of application rate of Paclobutrazol on
enhancement of flowering and fruit production in mango and to study the
efficacy of Thiourea as a dormancy breaking chemical to induce floral bud
break in mature shoots of selected mango cultivars.

MATERIALS AND METHODS

Experiment I

A field experiment was conducted at Government Seed
Production Farm, Maha Illuppallama, to study the effect of rate of
application of Paclobutrazol on flowering of mango cultivars Velleikolomban and Willard. Three rates of Paclobutrazol (0.00, 0.50 and 0.75 g a.i. /m of
canopy diameter for cv Willard and 0.00, 0.75 and 1.25 g a.i. /m for cv Velleikolomban) were tested. The formulation used was a wettable powder with
10% Paclobutrazol. The amount to be applied to each tree was dissolved in 1.5
l of water and applied as a collar drench around the base of tree at about
one month after leaf bud break when the leaf flush was fully expanded.

Experiment was
conducted in a RCBD with six single-tree replicates. Trees were 8 years old.
Experiment was blocked according to tree size as measured by stem girth at 30
cm above the ground. Mango trees selected for this experiment had more than
80% of the shoots in the canopy as new flush.

Flowering was
assessed as the % of the shoots flowering using randomly selected 20 tagged
shoots. Panicle length was measured after full bloom using three panicles
taken at random. Data collected were analyzed by analysis of variance in
RCBD. For treatment mean comparison LSD values were calculated (P=0.05).

Experiment
II

An observation
study was conducted in farmers’ fields at Anuradhapura to assess the effect
of Paclobutrazol on mango productivity. Paclobutrazol was applied as a soil
drench when the major leaf flush fully expanded at the rate of 1.00 g a.i/m
for mango cultivars Karuthakolomban and Velleikolomban and 0.75 g a.i/m for
cv Willard. Five trees of Karuthakolomban and three trees each of Velleikolomban
and Willard were treated with Paclobutrazol. An equal number
of trees were used as controls. Trees used for this experiment was 12 years
old. After flowering, Imidacloprid (10 ml/10 l water) was sprayed to flower
buds before full bloom to control Mango Hopper damage to flowers. Total
number of fruits harvested in the following season was recorded and mean
number of fruits per tree and standard deviation calculated for each
treatment in each variety.

Experiment
III

To study the effect of Thiourea on floral bud break of
mango, an observation study was conducted at FCRDI, Maha Illuppallama, using
14 mango varieties. An aqueous solution of 0.75% Thiourea was sprayed to half
of the canopy of selected trees using a modified knapsack sprayer. Trees
selected for this experiment had mature shoots suitable for flower induction.
Flowering response was recorded up to four weeks after application of
Thiourea.

RESULTS AND DISCUSSION

Mango trees treated with Paclobutrazol showed early bud
break about two- weeks before the control trees. Flowering intensity in terms
of the percentage of shoots flowered and lengths of flower panicles are shown
in Table 1. Paclobutrazol enhanced the flowering intensity of both mango
cultivars. There was no difference in the level of flowering between the two
rates of application used in both varieties. It is a common phenomenon that
with flowering, some of the shoots of mango canopy produce leaf flush. It was
noted that trees treated with Paclobutrazol had only a few shoots with new
flush.

Paclobutrazol
shortened panicle length significantly (Table 1) and the compacted flowers
were visibly noted in treated trees. However, the rates used did not compact
the flowers too much. Shorter flower panicles give an indication of the fact
that Paclobutrazol has been taken up by the trees. Winston (1992) showed that
panicle size reduction is due to effect of Paclobutrazol, but not caused by
an increase in number of panicles. Excessively compacted panicles may fail to
set or hold fruit. Moreover, such excessively compacted panicles are more
susceptible to insect and pest attacks.

Number of fruits produced in mango cultivars Karuthakolomban,
Velleikolomban and Willard were higher when trees were
treated with Paclobutrazol (Table 2). Variation in number of fruits produced
per tree was also lower with Paclobutrazol application. In all three
varieties, flower bud break commenced one to two weeks before the flower bud
break in control trees that were in close proximity to treated trees. This
shows that Paclobutrazol promote early flowering as well. These results agree
with numerous studies where promotion of early and profuse flowering and
higher yields in mango with Paclobutrazol has been reported (Burondkar and
Gunjate, 1991; Dissanayake, 1989; Kulkarni, 1988; Peiris, 2000; Tongumpai et
al., 1991; Winston, 1992).

Observation
study conducted to ascertain the efficacy of Thiourea as a mango flower
inducer showed that out of 14 varieties tested, four varieties i.e. Carabao,
Kohu-amba, Nam Doc Mai and Willard responded positively with flowering (Table
3). Carabao shoots showed the earliest bud break 11 days after Thiourea application while Kohu-amba flowered 22 days after treatment. Repeating the
experiment with those responsive varieties for three years confirmed that
Thiourea could be effectively used to induce floral bud break in mature
shoots of these mango varieties.

Unpredictable flowering, yield and production are due
primarily to unnecessary growth flushes that emerge as a result of various
environmental stimuli at various times during flush maturity. Such additional
leaf flushes result in immature shoots on the tree during the period of
natural flowering. It is noted that when Paclobutrazol is applied after the
leaf flush is fully expanded, it can effectively control emergence of
unwanted leaf flushes until the shoots are fully mature and ready to flower.

When the
shoots are fully mature, it is necessary to break dormancy of shoots to
stimulate floral bud break, as and when the grower desires to get flowers. As
shown in this study, Thiourea can be used for this purpose in some mango
varieties. Further studies are required to determine if other varieties
respond with higher concentrations of Thiourea or with modified application
methods. If the inactivity is due to a failure in chemical uptake, an additive
to increase Thiourea uptake might be advantageous.

Chemicals such
as aqueous solutions of potassium nitrate, ammonium nitrate and calcium
nitrate may also be used to stimulate bud break and needs to be tested with
varieties that do not respond to Thiourea. However, these chemicals are
explosives and therefore should be handled with care. Moreover, as shown in
the literature, higher concentrations are required for the above chemicals.
Therefore, safety and cost considerations must also be taken into account
when using such chemicals.

A package of technology for off-season mango
production needs 1. A technique to get the leaf flush mature for about 4-5
months without additional flushes and 2. A technique to induce floral bud
break in mature shoots. Paclobutrazol can be effectively used as a valuable
tool to suppress the growth of additional flushes in order to promote profuse
flowering in mango trees. Thiourea works with some mango cultivars as an
efficient dormancy breaker and cause floral bud break in mature mango shoots
and therefore it can be used to get flowering on trees as and when the grower
desires.

CONCLUSIONS

Paclobutrazol applied as a color drench to mango
cultivars Karuthakolomban, Velleikolomban and Willard after the major leaf is
flush fully expanded increased flowering intensity and number of fruits per
tree. Thiourea can be used as a 0.5% aqueous foliar spray to induce flower
bud break in mature mango shoots in some mango cultivars. These preliminary
results suggest that these two growth regulators can be used to control
growth, maturity and flowering of mango shoots that is an important
prerequisite for off-season production of mangoes. Further research is
required to fine tune this technology and to apply the same for other mango
cultivars grown in Sri Lanka.